Method for underwater transportation and installation or removal of objects at sea

ABSTRACT

Method for transport and installation of objects at sea, particularly relating to transport and installation of obs jects that are part of the infrastructure in oil and gas fields offshore, the object ( 4 ) is put in the sea at a suitable location near the shore or in sheltered waters, then towed to the installation site while being suspended in a slender buoyancy unit ( 5 ) acting much like a heave compensating unit. Upon arrival at the installation site, the suspension of the object ( 4 ) is transferred from the buoyancy unit ( 5 ) to a heave compensated winch ( 3 ) on a surface vessel ( 2 ), preferably the same vessel as used for the preceding towing operation. The winch is used to lower is the object ( 4 ) to its destination on the sea bottom or a predetermined location above the sea bottom.

RELATED APPLICATIONS

The present application is a National Phase application ofPCT/N003/00078 filed on Mar. 6, 2003 and publish in English, whichclaims priority from Norwegian Patent Application No. 20021119 filed onMar. 6, 2002, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

Oil and gas field developments are experiencing a push towardsunderwater production with more of the infrastructure placed on theseabed. There is thus an increasing need for transportation of objectswith subsequent installation of the objects at the field. Also, with anincreasing number of oil and gas fields being decommissioned, there is agrowing need for removal of objects with subsequent transport to shore.Some of the objects that are to be installed or removed from theoffshore sites are relatively complicated with large dimensions andweights. One is often dependent on costly vessels and equipment, and theavailability of such vessels may also be poor. Based on these aspectsthere is a need to develop new and alternative methods for transport andinstallation/removal of objects, as conventional methods become unfit,inadequate or very expensive.

The more conventional methods are normally based on transporting theobjects to the destination on deck of the installation vessel or atransportation barge, with subsequent offshore lift from deck andlowering of object through the splash zone/sea surface using a crane.Such operations set high demands to crane capacity and deck space, andcan be very weather sensitive operations depending on type of object tobe installed and the motion characteristics of the installationvessel(s). They further require costly construction vessels andpotentially tying them up for long periods of time depending on weather.

Thus, the present invention, in a first aspect, relates to a method fortransport and installation of objects at sea, particularly relating totransport and installation of objects that are part of theinfrastructure in oil and gas fields offshore, where the object is putin the sea at a suitable location near the shore or in sheltered waters,then towed to the installation site while being suspended in at leastone floating buoyancy unit, and subsequently lowered to its finaldestination.

A method of this type is known from GB 1191146. In this method twoslightly different types of buoys are used for suspending a pipelinewhile it is being towed from the shore to its place of installation. Atthe installation site, the pipeline is lowered to the sea bottom byflooding the buoys, some of them deliberately and others automaticallythrough the implosion of bursting disks so as to fill their buoyancychambers at a predetermined depth. The lowering of the pipeline isstarted from one end, and if the water is deep, a substantial part ofthe pipeline will be suspended in the still floating part before thefirst end reaches the bottom, thus subjecting part of the pipeline tovery substantial bending loads and possibly also causing the loweringprocess to become out of control. GB 1191146 is silent on the possiblerecovery of the buoys after the pipeline has reached the bottom. In anycase, such recovery work would be quite complicated, particularly atgreater depths.

U.S. Pat. No. 4,690,586 discloses a method for installing the jacket ofan offshore platform structure, wherein the jacket is towed on a bargeto the installation site. At the site the barge is submerged by means oftension legs and ballasting so that the jacket may be pulled off thebarge while in floating condition. This method is only possible forobjects that have sufficient buoyancy to be self-floating and cantherefore not satisfy the first aspect of the present invention asmentioned above. Besides, the installation of temporary tension legs forthe barge adds considerably to the complexity and cost of this method,which also seems limited to shallow waters and good weather conditions.

According to a second aspect, the present invention also relates to amethod for removal and transport of objects at sea. In this respect, US2001/0053311 discloses the use of a sea going crane vessel for raising ajacket structure or the like to a horizontal position so that it may besuspended at either end of a barge. The suspension points on the bargeare releasable rocker beams in order to facilitate quick release of thejacket once it has been towed to a place where it can be dumped at deepwater. This method is limited to water depths where jackets may be usedand where the object may be raised one end at a time. The dimensions ofthe barge must correspond approximately to the length of the object. Thebarge will have a substantial water line cross-section and thereforerespond to wave motion, and with the object suspended in releasablerocker beams it will not be possible to lower the object in a controlledmanner after its release.

Thus, the purpose of the present invention is to provide a method asdefined in the preamble of claims 1 and 2, respectively, which is safeand simple and may be performed with the use of readily availableequipment even when the weather conditions are not favourable.

SUMMARY OF THE INVENTION

The transport and installation method may be summarised as follows:

The object is transported from the fabrication yard to a suitablelocation inshore or a location in sheltered waters or a location nearshore with the appropriate weather to perform the transfer operations.An inshore crane vessel will then pick up the object from thetransportation barge or transportation vessel and lower the objectthrough the splashzone/sea surface with subsequent hook-up and weighttransfer to the buoyancy unit and towing arrangement, according to theinvention. The object will then be towed to the destination by a towingvessel while immersed and suspended from the buoyancy unit. On arrivalat the destination the towing winch wire from the towing vessel will beshortened to take the weight of the suspended object and enable releaseof the buoyancy unit. The buoyancy unit will be released from the objectwhen it no longer carries any weight, and subsequently stored on deck ofor alongside of the towing/installation vessel, or other vessel, withpost-installation transport back to shore either on deck or towed in ahorizontal position, respectively. The object will then be lowered tothe seabed or to a specified target depth between the surface and theseabed for installation with the heave compensated towing winch wirefrom the towing vessel or other vessel with heave compensated liftingarrangement.

This is obtained by a method as recited in claim 1.

The invention also relates to removal operations as defined in claim 2.

When applying the invention one achieves a number of advantages comparedto conventional methods. In particular, one avoids the problems andweather restrictions associated with an offshore lift from deck andsubsequent lowering through the splashzone/sea surface. Avoiding usingcrane vessels and improving the weather criteria for the installationwill also result in a major reduction in cost for the operation, and theinvention is in principle independent of dimensions and weight of theobject to be transported and installed. Further, the transport todestination is safer and less weather sensitive since the object issuspended from a slender buoyancy unit and is independent of theinstallation vessel during transport. The buoyancy unit has a small andconstant waterplane area and acts as a heave compensator, therebyreducing the dynamic loads in the object and the suspension arrangementbetween the object and buoyancy unit. The method can be based on usingthe same vessel for both the tow and the installation or recovery, and aminimum of equipment is needed. This means that any vessel thatsatisfies the basic capacity requirements may be used, which increasesvessel availability, opens for using low cost vessels and reduces vesselmobilisation time.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention shall be described in more detail in the followingwith reference to the attached schematic drawings which illustrate apreferred embodiment, wherein:

FIG. 1 is a side view showing how a typical inshore crane vessel liftsthe object from deck of a vessel or a transportation barge and lowers itinto the sea for connection to the towing vessel.

FIG. 2 is a plan view showing the connection of the buoyancy unit andthe towing wire to the suspension arrangement while the object ishanging below the stern of the towing vessel and the buoyancy unit lyingon deck of the towing vessel.

FIGS. 3A–F are side views illustrating the launch of the buoyancy unitwith subsequent weight transfer from towing winch to buoyancy unit.

FIG. 4 is a side view illustrating the invention during the transportphase, with an object suspended from a slender buoyancy unit andconnected via a towing arrangement to a towing vessel.

FIGS. 5A–F are side views illustrating the end of tow situation wherethe object weight is transferred from the buoyancy unit to the towingwinch, with subsequent disconnection and recovery of the buoyancy unit.

FIG. 6 is a side view showing the lowering of the object towards seabedor target depth for installation.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows the start of the operation according to the invention. Theobject 4 is being lifted through the surface 1 into the sea in shelteredwaters after having been lifted off the deck of a vessel ortransportation barge by a crane vessel 12. The object will be connectedto the lower part of the suspension arrangement 7 and hung off in sharkjaws at the stern of the towing vessel 2. The lifting wire from thecrane vessel is then disconnected from the object.

FIG. 2 shows the object 4 hanging just beneath the stern of the towingvessel 2, hung off in the shark jaws 14. The buoyancy unit 5 is lying ondeck of the vessel and is connected to the tri-plate 8. The towing winchwire 10, running from the towing winch 3 via a heave compensator 13 ondeck, is also connected to the tri-plate.

FIGS. 3A–F show the launch of the buoyancy unit 5 from the towing vessel2 and the subsequent object 4 weight transfer from the towing winch 3 tothe buoyancy unit. In FIGS. 3A–C the towing winch wire 10 is paid outand the buoyancy unit 5 is thereby pulled off the deck by means of theobject 4 weight. This launch operation is performed with the vessel 2 inforward motion. In FIGS. 3D and E the towing wire continues to be paidout and the buoyancy unit starts to take some of the object weight andis thereby raised towards a vertical position. In FIG. 3F the buoyancyunit 5 has taken the full weight of the object 4.

FIG. 4 shows the towing situation of the present invention wherein thetowing vessel 2 is an anchor handling tug with an onboard heavecompensated winch. Further, the object 4 is shown suspended from thecylindrical buoyancy unit 5, which are connected to each other by asuspension arrangement 6, 7, 8. The winch wire from the anchor handlingtug, or other heave compensated winch, is used as towing wire 10. Thetowing wire is connected to a buoy 11, which relieves the buoyancy unitof the weight of the towing wire. If required, the directional stabilityof the towed object 4 may be controlled by a guideline, denoted 9.

The buoyancy unit 5 has a long, slender, cylinder shape with a small andmainly constant waterplane area and a tapered lower end. The unit willpreferably be a steel structure that may be divided into severalwatertight compartments, a principle that will ensure continued buoyancyin case of leakage or damage to one or more of the compartments. In thelower tapered end of the buoyancy unit there may be a padeye arrangementfor connection of the suspension arrangement 6–8 between the suspendedobject 4 and the buoyancy unit 5. The buoyancy unit may also be fittedwith a ballast system which, as required, can be utilised to adjust thevertical position of the buoyancy unit 5 in the water, and also enableconnection or disconnection of a liftline between the buoyancy unit andthe object.

The main purpose of the buoyancy unit 5 is to keep the towed object 4afloat and in the capacity of its shape act as a heave compensator andthereby minimise the dynamic loads in the towed object 4 and thesuspension arrangement 6, 7, 8 between the object and the buoyancy unit5. The heave compensation achieved by this principle is not limited bye.g. a defined cylinder stroke length, as large waves will wash over thebuoyancy unit. Thus, the resulting changes in dynamic loads will besmall in the towed object 4 and the suspension arrangement 6–8 betweenobject and buoyancy unit.

The suspension arrangement 6, 7, 8 between the slender buoyancy unit 5and the object may consist of two parts 6, 7 connected together by alink 8, preferably a tri-plate. The purpose of dividing the arrangementin such a manner is to be able to pull the tri-plate 8 onto deck forconnection and disconnection of the towing wire 10 to the suspensionarrangement, while the object 4 is still hanging below surface 1.

The actual towing force is acting in the link 8 between the upper 6 andlower 7 part of the suspension arrangement, in such a way that neitherthe towed object 4 nor the buoyancy unit 5 is directly connected to thetowing wire 10.

FIGS. 5A–F show the end of the tow when the destination is reached, withweight transfer from buoyancy unit 5 to the towing winch 3 andsubsequent recovery of the buoyancy unit. In FIGS. 5A–D the towing winchwire 10 is hauled in to take the weight of the object 4. The buoyancyunit will gradually take less of the object load until it is floatinghorizontally on the surface without carrying any load. In FIG. 5E thebuoyancy unit is disconnected from the object 4 and a recovery winch(not shown) is attached to it for recovery of the buoyancy unit ontodeck of the towing vessel 2. In FIG. 5F the buoyancy unit is lying ondeck of the towing vessel while the object is hanging in the winch wireready for deployment towards seabed.

FIG. 6 shows the object 4 being lowered towards seabed for finalinstallation. A clump weight and orientation wires (not shown) aretypically used as installation aids for positioning of the object at theseabed.

The vessel 2 used for towing the object 4 will according to theinvention preferably also be used for installation of the object uponarrival at the destination, as the object 4 then is lowered to theseabed using the towing winch wire 10. However, the method according tothe invention is not limited to using the same vessel for transport andinstallation, and the method according to the invention could thusinclude using more than one vessel. The vessel performing theinstallation or recovery offshore may be any type of vessel,self-propelled or not, with a heave compensated lifting arrangement.

The method is intended for use with only one slender buoyancy unit 5 fortransport and installation of relatively compact objects where thestructural design is such that it can be lifted or carried in a singlepoint using a lift bridle or similar. Moreover, the method may be usedwith two or more equivalent slender buoyancy units when the object has astructural design that requires two or more points for lifting/carryingto ensure the integrity of the structure during the transport andinstallation, according to the Invention, e.g. for transport andinstallation of large pipe spoolpieces.

In relation to spool piece installation, the method according to theinvention is considered not only applicable for installation of singlespoolpieces, but is also applicable for transport and installation ofseveral spoolpieces simultaneously. This is made possible by having anumber of spoolpieces stacked, or in other way placed, in a basket ortransportation frame that will be suspended from the buoyancy unit asper the method according to the invention. The transportation frame maythen be lowered to the seabed and wet parked on arrival at thedestination, and the spoolpieces may then be picked up from the basketone by one and installed.

In addition to transport and subsequent installation of objects, themethod according to the invention is also suitable for removal ofobjects and transportation of these to shore. This is beneficial when afield is to be decommissioned or when objects are to be removed forother reasons, e.g. repair or replacement. Some objects are also unfitfor recovery to the surface and lifting onto deck of a constructionvessel or a transportation barge due to cost or technical limitationslike large object dimensions and/or weight. The step by step method forremoval and subsequent transport to shore of an object will in principlebe the reverse of the step by step method already outlined forinstallation of an object. It is envisioned that removal of structuresusing the method also may include e.g. removal of jackets, as these maybe laid down on the seabed for subsequent recovery and transport toshore using the method according to the invention. The removal method isdefined in claim 2.

The method may also be used to install objects that in operation are tobe located mid-water, i.e. in a position below the sea surface, betweenthe seabed and the critical wave zone. Such an object is e.g. anArtificial Buoyant Seabed (ABS), also referred to as Atlantis. Themethod according to the invention is in this respect not limited toinstalling or recovering objects on the seabed, but also involvesinstalling or recovering objects from mid-water positions.

Another possible utilisation of the method is to have at least one winchmounted on the buoyancy unit itself, to be used for controlled loweringof the object towards seabed or target depth between seabed and seasurface. Due to its shape, the buoyancy unit acts as a heave compensatorand thereby minimises the dynamic loads in the winch wire, liftarrangement and object during the lowering and final landing, and makesthe lowering and landing operations independent of installation vesselmotions. Alternatively, the same advantages may be gained by having asheave arrangement or similar mounted on the buoyancy unit itself, withat least one sheave or similar. At least one winch from at least onevessel may then be routed over the sheave arrangement and furtherconnected to the object for lowering and final landing of the object.These utilisations of the method may also be used for recoveryoperations.

If the tow will encounter more shallow depths underway where thesubmerged object 4 could possibly hit the sea bottom, it is envisionedthat when passing such waters, the object 4 is raised somewhat bybringing the buoyancy unit into a horizontal position. This could bedone in various ways, e.g. by connecting a wire between the triplate 8and the top of the buoyancy unit and then shortening this wire until ithas the same length as the upper suspension wire 6.

The invention is not limited to the exemplifying embodiment describedherein, but may be varied and modified by the skilled person within thescope of the appended claims.

1. A method for transport and installation of objects at sea,particularly relating to transport and installation of subsea objectsthat are part of the infrastructure in oil and gas fields offshore,where the object is put in the sea at a suitable location near the shoreor in sheltered waters, then towed to the installation site while beingsuspended via a suspension arrangement of a predetermined length in atleast one floating buoyancy unit, and subsequently lowered to its finaldestination, wherein use is made of at least one slender uprightbuoyancy unit, which also is used as a heave compensator during thetowing and thereby reduces the dynamic loads in the towed object and thesuspension arrangement between the towed object and the buoyancy unit,and that upon arrival at the installation site, the suspension of theobject is transferred from said at least one buoyancy unit to a heavecompensated winch on a surface vessel while the buoyancy unit isdisconnected, said winch being used to lower the object to itsdestination on the sea bottom or a predetermined location above the seabottom.
 2. A method according to claim 1, wherein the freeboard of theor each buoyancy unit is set so small that large waves will wash overthe buoyancy unit, whereby the resulting changes in dynamic loads willbe small in the towed object and the suspension arrangement betweenobject and buoyancy unit.
 3. A method according to claim 1, wherein theor each buoyancy unit has a constant diameter along the longitudinalaxis, has a tapered lower end, and is divided into several watertightcompartments to ensure continued buoyancy in case of leakage or damageto one or more compartments, and in the lower tapered end has a padeyearrangement for connection of the suspension arrangement between thesuspended object and the buoyancy unit.
 4. A method according to claim1, wherein the or each buoyancy unit is fitted with a system forballasting, for the purpose of adjusting the vertical position of thebuoyancy unit in the water and thus enable connection or disconnectionof a liftline between the buoyancy unit and an object on the seabed oran object in a position mid-water, or for the purpose of selecting thedraft and freeboard of the buoyancy unit in the upright position toadjust the motion behaviour.
 5. A method according to claim 1, wherein atowing arrangement is used comprising a towing line which connects thebuoyancy unit and object with a towing vessel.
 6. A method according toclaim 5, wherein the towing arrangement further comprises a guidelinebetween the vessel and object to control the directional stability ofthe towed object.
 7. A method according to claim 5, wherein the towingline in the towing arrangement is the same winch wire of the towingvessel winch system which is utilized for lowering or raising the objectduring installation or removal, respectively.
 8. A method according toclaim 1, wherein the suspension arrangement that connects the object tothe buoyancy unit comprises a lower part and an upper part connectedtogether with a link, preferably a tri-plate, to enable connection anddisconnection of a towing line to the suspension arrangement on the deckof a towing vessel.
 9. A method according to claim 1, wherein the vesselused for lowering or raising the object is also used for towing theobject.
 10. A method according to claim 2, wherein the vessel used forlowering or raising the object is also used for towing the object.
 11. Amethod for removal and transport of objects at sea, particularlyrelating to removal and transport of objects that are part of theinfrastructure in oil and gas fields offshore, where the object isrecovered from the seabed or a position between the seabed and seasurface, with subsequent tow to a predetermined place, wherein theobject is raised to a predetermined distance below the surface by meansof a winch on a surface vessel, whereupon the suspension of the objectis transferred via a suspension arrangement of a predetermined length toat least one floating buoyancy unit before the tow is started, whereinuse is made of a heave compensated winch during the raising, and furtherthat use is made of a slender upright buoyancy unit, which is also usedas a heave compensator during the towing and thereby reduces the dynamicloads in the towed object and the suspension arrangement between thetowed object and the buoyancy unit.
 12. A method according to claim 11,wherein the freeboard of the or each buoyancy unit is set so small thatlarge waves will wash over the buoyancy unit, whereby the resultingchanges in dynamic loads will be small in the towed object and thesuspension arrangement between object and buoyancy unit.
 13. A methodaccording to claim 11, wherein the or each buoyancy unit has a constantdiameter along the longitudinal axis, has a tapered lower end, and isdivided into several watertight compartments to ensure continuedbuoyancy in case of leakage or damage to one or more compartments, andin the lower tapered end has a padeye arrangement for connection of thesuspension arrangement between the suspended object and the buoyancyunit.
 14. A method according to claim 11, wherein the or each buoyancyunit is fitted with a system for ballasting, for the purpose ofadjusting the vertical position of the buoyancy unit in the water andthus enable connection or disconnection of a liftline between thebuoyancy unit and an object on the seabed or an object in a positionmid-water, or for the purpose of selecting the draft and freeboard ofthe buoyancy unit in the upright position to adjust the motionbehaviour.
 15. A method according to claim 11, wherein a towingarrangement is used comprising a towing line which connects the buoyancyunit and object with a towing vessel.
 16. A method according to claim15, wherein the towing arrangement further comprises a guideline betweenthe vessel and object to control the directional stability of the towedobject.
 17. A method according to claim 15, wherein the towing line inthe towing arrangement is the same winch wire of the towing vessel winchsystem which is utilized for lowering or raising the object duringinstallation or removal, respectively.
 18. A method according to claim11, wherein the suspension arrangement that connects the object to thebuoyancy unit comprises a lower part and an upper part connectedtogether with a link, preferably a tri-plate, to enable connection anddisconnection of a towing line to the suspension arrangement on the deckof a towing vessel.
 19. A method according to claim 11, wherein thevessel used for lowering or raising the object is also used for towingthe object.
 20. A method according to claim 12, wherein the vessel usedfor lowering the object is also used for towing the object.